In many applications, it is desirable to detect the presence of ions in a chamber or space. For example, in a mass spectrometer, ions of the various gas constituents are detected to determine the partial pressure of each gas constituent in a chamber and compared to the detected total pressure of the gas within the chamber. By detecting the partial pressure of each particular gas constituent, as well as the total pressure of the combined gases within the chamber, useful information can be acquired. For example, both total and partial pressures are proportional to the corresponding volumetric number density, respectively, of the total and constituent gases, thus providing information of the quantity of each gas constituent that is present. Knowledge of total and partial pressures is useful, for example, for detecting leaks in a system. For this and other reasons, it is highly desirable to measure both total and partial pressures as accurately and precisely as possible.
In conventional mass spectrometers and other systems, measurement of the partial and total pressures of the gases is based upon the probability of an electron colliding with a neutral atom or molecule, and thereby creating a positive ion. The probability is proportional to the volume number density of the neutral atom or molecule along the electron flight path. The probability is a function of the partial and total pressures, with the probability increasing with increasing pressure. Ions thus are measured within the chamber. In quadrupole mass spectrometers, partial pressures are measured using a quadrupole mass filter assembly and an ion current measurement device, positioned at the output of the filter assembly within the chamber, having a surface which (a) is exposed to the ions exiting the filter, and (b) generates a current when positively ionized particles contact a surface of the device. A current measurement instrument is used to measure the current which is proportional to the total volumetric number density of the neutral atoms or molecules of the gas constituent being measured, and therefore is proportional to the partial pressure of the neutral atoms or molecules of that gas. Thus, knowledge of the current due to ions contacting the surface provided with the current measurement instrument provides knowledge of the partial pressure of each constituent gas. Typically, the surface provided with the current measuring instrument is an ion detector which includes a device commonly known as a Faraday plate or cup. Charged ions strike the Faraday plate causing an ion current to be generated in the plate.
The Faraday plate is useful for detecting ions at relatively high chamber pressures, and in fact a second Faraday plate or cup can be used in the chamber to measure the total pressure in the chamber by continually detecting positive ions created in the chamber from all of the constituent gases. However, at low pressures, where the ion current is low, it is often desirable to enhance the sensitivity of the ion detector. One solution is to detect the ions with an electron multiplier. Electrons produced by the multiplier are collected by an anode or electron collector. Current at the anode is measured to quantify the electrons and to indicate the input ion current.
More specifically, an electron multiplier typically includes an ion/electron converter typically comprising a layer of doped resistive material. Electrons emitted from the converter in response to detected ions are increased (or multiplied) by a predetermined factor so as to create additional or secondary electrons measured through a more easily detected dynamic range. The space in which the number of electrons are multiplied is typically subjected to a bias voltage applied across the length of the multiplier space. The bias voltage creates an electric field gradient. Ions from the chamber enter the multiplier and strike the surface of the ion/electron converter, resulting in the release of electrons from the surface. Additional or secondary electron generating surfaces are provided within the field gradient so that when an electron travels through the field gradient and strikes one of these surfaces, there is a high probability that multiple secondary electrons are generated from the surface for each electron that strikes the surface. These secondary electrons are accelerated by the electric field such that they in turn strike another internal surface to cause the release of more secondary electrons, and so on. Finally, the secondary electrons exit the multiplier and strike the anode. The current at the anode is measured to quantify the electrons exiting the multiplier. In principle, since the gain of the multiplier, i.e., the number of electrons exiting the multiplier for each ion entering, is known, the number of electrons measured provides a determination of the number of ions and, therefore, the measured pressure. The predetermined factor or gain of typical electron multipliers used in presently available mass spectrometers typically varies from as low as 1000 to as high as 10,000,000.
The gain of the multiplier is determined by several of its characteristics and operating parameters, including the multiplier geometry and composition and the applied bias voltage level creating the electric field gradient. Given a particular multiplier, the gain is controllable by varying the bias voltage so as to vary the electric field gradient, although in the prior art it is assumed that the gain remains fixed during operation of the electron multiplier. Ideally, the gain of the multiplier is independent of pressure in the chamber. However, certain phenomena that occur within the multiplier cause the gain to vary with chamber pressure. One such phenomenon is referred to as ion feedback, which causes the gain to increase rapidly with increased pressure, particularly at high gain.
Ion feedback occurs when one or more of the secondary electrons inside the multiplier strike gas molecules with sufficient energy to ionize them. The resulting ions and electrons are accelerated by the electric field within the multiplier until they collide with an internal surface, causing more secondary electrons to be released and to produce still more secondary electrons. The result is more electrons exiting the multiplier for a given gain (bias voltage).
At low pressures, very few gas molecules are present in the multiplier and, therefore, the relatively small effects of ion feedback are negligible. However, at higher pressures, many more gas molecule collisions take place, and the gain of the multiplier varies rapidly with chamber pressure. Significant ion feedback typically occurs when the pressure at the electron multiplier is above 1.0 millitorr. As a result, the electron current measurement taken at the output end of the multiplier no longer provides a reliable measurement of the number of ions entering the multiplier, and inaccuracies are introduced into the pressure measurement. Further, operating at very high gains and high pressures increases the chances of voltage discharge and/or breakdown, as well as decreases the useful life of the multiplier by increasing the number of collisions with the doped inner surfaces of the multiplier. For this reason, electron multipliers of the prior art typically are not operated when the pressure at the electron multiplier is above 0.5 millitorr.
Presently, there are quadrupole mass spectrometers designed to operate at pressures up to about 20 mtorr. At least one of these spectrometers uses a Faraday cup ion detector, which as described above, does not have good performance at very low pressures. At least another of these prior art spectrometers includes both an electron multiplier with a collection anode and a Faraday cup to detect ions in a mass spectrometer. As a solution to the dependence of gain on gas pressure, this prior art system uses the electron multiplier for low pressures and the Faraday cup at high pressures. Specifically, at low pressures, ions entering the electron multiplier are multiplied as described above, and the electrons produced thereby are collected by the anode. The anode current is measured. As the pressure increases beyond a predetermined threshold (the threshold being equal to or less than 1.0 mtorr), within the 1.0-20.0 mtorr range, the multiplier is not used, but instead the ion current is measured directly with the Faraday cup. In such a system, the low-noise amplification benefits of the electron multiplier are forfeited at these higher pressures.